Gypsum is a common material that is widely employed in the building industry for a number of uses. The usefulness of gypsum is due, at least in part, to its reasonable cost and the ability to form it into substantially any shape. Gypsum is also known as calcium sulfate dihydrate, terra alba, or landplaster and is generally produced by mining or isolation from flue gas desulfurization processes occurring at power plants. In its raw form, gypsum is found as a dihydrate and has approximately two water molecules associated with each molecule of calcium sulfate.
Generally, gypsum-containing products are prepared by forming a slurry of calcined gypsum (calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water, and optionally other components, as desired. Typically, as its name suggests, calcined gypsum is produced by calcining calcium sulfate dihydrate to drive off some of the water of hydration. To form a three-dimensional plaster object, the calcined gypsum slurry typically is cast into a pre-determined shape or applied onto the surface of a substrate where it can be machined into a pre-determined shape. Once cast or applied to a substrate, the calcined gypsum reacts with the water to form a matrix of crystalline hydrated gypsum. It is this hydration that enables the formation of an interlocking matrix of set gypsum, thereby imparting strength to the plaster product. Mild heating can be used to drive off any unreacted water to yield a dry product.
While castable and/or sprayable plaster compositions have been used in the prior art to produce three-dimensional objects, such methods are often not ideal. For example, cast plaster objects cannot readily be formed into complex three-dimensional shapes. The casting process also requires special tooling such as molds and dies and it can be a slow and cumbersome method for the production of three-dimensional objects. In addition, plaster objects are often subjected to sustained elevated temperatures (e.g., 140° F. or higher) during the casting process. This can be problematic because many conventional plaster compositions are thermally and dimensionally unstable to some degree. More specifically, surface re-calcination of the set gypsum can occur at elevated temperatures, thereby resulting in a loss of strength due to a weakened set gypsum matrix.
Similarly, sprayable plaster compositions cannot readily be formed into complex three-dimensional shapes (e.g., shapes with internal structure) due to the requisite substrate coating and machining processes. In this regard, sprayable plaster compositions require laborious and inefficient manufacturing steps before even rudimentary three-dimensional objects can be produced. In particular, a substrate must first be fabricated that is similarly shaped but slightly smaller than the desired full-size object. The substrate can then be coated with a sprayable plaster composition to form a substrate/plaster combination which is larger than the desired full-size object. Finally, the coated substrate must then be machined (e.g., cut, milled, lathed, sanded, etc.) into the desired shape. As such, a large quantity of the plaster material is unavoidably wasted as plaster dust and/or shavings during the manufacturing process.
Accordingly, there remains a need for improved methods for the production of three-dimensional plaster objects that address the shortcomings of the processes discussed above and known in the prior art. The present invention provides such methods. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.